In IP (Internet Protocol) network, network diagnosis tools such as ping/trace route are used to diagnose the accessibility of a target network, the routing situation of an IP package, and the position of failure in routing. The message of Internet Control Message Protocol (ICMP) is used to transmit control information between a router and a host, and FIG. 1 shows PDU format of the existed ICMP, comprising type, code, checksum, and option data. A network administrator can use these control information to diagnose route problems in the network.
Two commonly used route diagnosis tools are respectively introduced as follows. One is ping tool, and its processing is as follows: a source website (i.e. a router or a host) sends several ICMP echo messages to a target website, if the target website receives these ICMP echo messages, an ICMP reply message is used to reply them, and the source website diagnoses whether the target website can be accessed and whether the transmission is delayed according to the ICMP reply message; FIG. 2 shows the PDU format of the existed ICMP echo message, wherein, the specific contents of the PDU include identifier, sequence number, and data. The other is trace route tool, and its processing is as follows: a source website sends to a target website an ICMP message or a UDP message whose time to live value (i.e. the TTL value used for calculating how many routers a data package has passed through) increases progressively from 1, and a middle router takes turns to loopback an ICMP timeout message until reaching the target website because TTL is timeout; the target website replies the ICMP reply message or an ICMP port unaccessible message and takes turns to record source addresses of the corresponding messages of the ICMP. The delivery routing of the IP package can be shown by using trace route tool.
However, all the above mentioned diagnosis tools such as ping/trace route can only be applied to a network solely based on target-address-routing, if the IP package, during being delivered, passes through a certain apparatus supporting policy-based routing, the routing of a diagnosis-protocol-message is very likely to be different from that of a service stream being diagnosed, and then different diagnosis results will be obtained. A network system as shown in FIG. 3, Host A and Host B are personal computers, and Routers A-E are routers supporting policy-based routing. Host A can employ ping to check the route accessibility between Host A and Host B, and according to the existed ICMP technique, suppose the path of the message route routing is: Host A→Router A→Router B→Router C→Router E→Host B. If the TCP connection from Host A to Host B is policy-selected-routing to Router D at Router B, for example, a certain Access Control List (ACL), then the results of the above mentioned ping cannot truly show the accessibility of the TCP message from Host A to Host B, that is to say, there may exist a situation that Host A can communicate with Host B by ping, but the TCP message of Host A cannot reach Host B.